Opens a file indicated by filename and returns a file stream associated with that file. mode is used to determine the file access mode.

Parameters

filename

-

file name to associate the file stream to

mode

-

null-terminated character string determining file access mode

File access mode string

Meaning

Explanation

Action if file already exists

Action if file does not exist

"r"

read

Open a file for reading

read from start

failure to open

"w"

write

Create a file for writing

destroy contents

create new

"a"

append

Append to a file

write to end

create new

"r+"

read extended

Open a file for read/write

read from start

error

"w+"

write extended

Create a file for read/write

destroy contents

create new

"a+"

append extended

Open a file for read/write

write to end

create new

File access mode flag "b" can optionally be specified to open a file in binary mode. This flag has effect only on Windows systems. On the append file access modes, data is written to the end of the file regardless of the current position of the file position indicator.

When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end- of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.

File access mode flag "x" can optionally be appended to "w" or "w+" specifiers. This flag forces the function to fail if the file exists, instead of overwriting it. (C11)

When using fopen_s or freopen_s, file access permissions for any file created with "w" or "a" prevents other users from accessing it. File access mode flag "u" can optionally be prepended to any specifier that begins with "w" or "a", to enable the default fopen permissions. (C11)